Solder bumps have become a standard way to connect semiconductor devices to substrates and substrates to the Printed Circuit Board (PCB). It is therefore, that the technology is important for two different industries, Wafers fabrication and PCB manufacturing. Although there is a lot of similarity between PCB and wafer bumps, there are also differences in height and diameter of the bumps, which affect the process control. In general, wafer bumps are substantially smaller than PCB bumps. Typical numbers are 100 μm bump height in wafers and 300 μm in PCB, but there can be large variations from these numbers.
To have a good connection, all bumps (there may be thousands of bumps on each die) must be uniform in their height within an allowable tolerance. Therefore, bumps height metrology is an essential part of the process control in both fabrications.
The common way to measure bumps in the PCB industry, is to scan over the area of an object (PCB or Substrate) with laser line triangulation. Triangulation has a long history as a 3D measurement tool, and it comes with a large variety of optical configurations. U.S. Pat. No. 3,187,185 (to Milnes and Pitcairn, June 1965) describes an optical triangulation system in which a narrow strip of light is projected upon a flat object from a well-defined angle α. A camera, images the object from another defined angle β. The height of the object can then be measured from the position of the strip upon the image, provided that angles α and β are known and the object is considered to be flat. In a second embodiment, Milnes and Pitcairn also present a configuration of two line projectors and one camera. This concept is the basic configuration for most bumps metrology systems today. As a light source it is common to use a laser, because it provides high intensity at a very narrow strip and the spatial coherence of the laser guarantees good definition of the illumination angle. U.S. Pat. No. 5,028,799 to Chen et al. describes a laser triangulation system for bumps metrology, employing two lasers and one camera. Two lasers can overcome several issues including different optical behavior of the solder and the substrate.
While laser triangulation provides satisfactory measurement of PCB solder bumps, it makes an excessive error measuring small Wafer bumps. This error is known as “shape-error”, and it is typical to small bumps or other features having smaller size than the illumination (the width of the strip). Although solder bumps are usually larger than 100 μm in diameter, since they have a ball shape, only a small section at the top of the ball can be observed. The nature of shape-error is that a feature will be measured properly only when located at the centerline of the strip. When the feature is moved forward or backward, the triangulation height measurement will have an up or down error respectively. While scanning, both errors will occur (up and down errors) therefore theoretically, they can be summed up to zero. This implies that an averaging along the scanning axis is highly recommended to improve shape-errors. Averaging alone however, can give satisfactory results in terms of accuracy (1-3 μm typically), only if the bumps have a smooth, well reflecting surface. In reality, the processes of manufacturing solder bumps create surface defects, especially if they are manufactured in a led-free process. These surface defects do not reflect light completely or partially, therefore they make it impossible to get enough data to eliminate the shape-errors by averaging. Large defect located at the bump-top can even prevent the measurement completely.
The present invention overcomes these disadvantages by using a spatially incoherent illumination in a unique optical configuration.
Another known optical-errors is expressed by the fact that usually different heights results are achieved from different direction of scanning. Therefore, a common way of measuring is to scan the object twice, from right to left and then from left to right and averaging the results. The “symmetrical configuration” of the present invention enables the averaging in a single scan.